I have a Max6675 thermocouple interface chip that I have had working with an
18F with no problem.  I've added two muxes (CD74HC4067) which are eight
channels each, so that I can multiplex the 6675 to 16 thermocouples.  Once I
threw the muxes into the mix, not unexpectedly the thermocouples read higher
temperatures at room temperature.  I suspect the "on" resistances of the
muxes (70 ohms) are having an effect.  Through some trial and error I came
up with a correction factor that I subtract from the 12-bit result of the
6675 which has moved the resultant temperature in line.  However, I
presently don't have a thermocouple calibrator to test across the
temperature range I am going to use the circuit in.  I've tested the
thermocouple readings by dipping the thermocouple in a cup of ice water and
a cup of boiling water, and the results are correct at those temperatures.
So my question is: will the additional impedance of the muxes have a linear
effect on the voltage produced by the thermocouples (which, by themselves
aren't linear) or should I expect the circuit to behave in a non-linear
fashion which would require a correction table that I will have to build?

Regards,
Bob
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